A variety of environmental insults induce the normally quiescent urothelium to mount a transient proliferative response and restore homeostasis. However, some insults, especially those with both potent mitogenic and mutagenic activities, lead to persistent urothelial proliferation that can progress to cancer, including urothelial (transitional cell) carcinoma, the most common cause of bladder cancer death in the United States. A better understanding of the events that promote bladder repair and influence the decision between a return to quiescence vs. prolonged growth and cancer formation could lead to improved therapies aimed at promoting normal growth or repair or controlling cancer. Based on extensive preliminary evidence, this application investigates the hypothesis that signaling by the Hedgehog (Hh) family of secreted morphogens promotes urothelial proliferation in response to chemical insults and becomes constitutively active in urothelial cancers. Mechanistically, we propose that Hh signaling in bladder injury repair is induced by expression of the 7 transmembrane span protein Smoothened (Smo), and activates several aspects of the malignant phenotype, including enhance proliferation, epithelial to mesenchymal transition (EMT), and cell motility. Since repair of tissue injury appears to be the proliferative stimulus resulting from exposure to chemical carcinogens, our findings suggest that urothelial carcinoma may originate as part of an epithelial repair program. To test this hypothesis, we propose a series of experiments to characterize the expression and roles of Hh pathway activity in urothelial homeostasis and carcinogenesis. The Aims to be pursued are: 1) To characterize the induction of Hh signaling, EMT, and stem cell activation in regenerating urothelium. 2) To test the effects of Hh pathway blockade on urothelial injury repair;3) To test whether Hh signaling is necessary for urothelial carcinoma growth. 4) To test whether continuously enforced Hh signaling is sufficient to induce urothelial carcinogenesis.